Constant-frequency oscillator



Jan. 28, 1930.

R. S. OHL

CONSTANT FREQUENCY OSCILLATOR Filed Sept. 25, 1925 !NVENTOR [3. 5. O/Ll/ATTORNEY Patented Jan. 28, 1930 UNITED STATES PATENT orrlcs aossnnr. s.can, or new You, n. 2, micron 'ro 1mm mnrrronn um um:-

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This invention relates to means fer reducing electrical oscillations andpart1 rly to a system of that type characterized by means to maintainsubstantially constant the frequency of the oscillations producedthereby.

In the production of electrical oscillations by means of vacuum tubedevices, dlfiiculty has been experienced in keeping constant thefrequency of said oscillations owing to the fact that the constants ofthe vacuum tubes affect the magnitudes of the tuned circuits, which inturn control the frequency. As is well known, these constants, such asthe plate impedance of a vacuum tube, are not the same during theoperation of the tube as when the tube is in non-operated condition, dueto changes within the tube itself as, for example, filament emission,gas pressure changes within the tube, and variations of plate voltage.

Various ways have been suggested heretofore to maintain constant thefrequency of such oscillators or to maintain constant the frequency inthe circuit supplied by such oscillators. One of the methods consists inusing a loose coupling between the oscillator and the antenna circuit.Another method consists in the use of a master oscillator, and a thirdmethod in the use of a crystal to control the frequency of a vacuum tubeoscillator, which frequency was then used in producinga harmonicfrequency, the harmonic being used as the radio frequency.

None of the schemes enumerated absolutely prevents a change of carrierfrequency, al though the master oscillator system eflects an improvementover the single oscillator tube, and to a greater extent the crystalcontrol system is better than the master oscillator system. Thesesystems, however, possess in herent defects, the most important beingthat they are diflicult to apply to the higher radio frequencies of theorder of a million cycles or more because of the number of tunedcircuits involved.

This invention resides in a system for the production of electricaloscillations, which is characterized by two tuned circuits, one of whichdetermines the frequency and the other drawing, of which Figure 1 showsschematically a simple form. of embodiment of the invention and Figs. 2and 3 are modifications of the form shown in Fig. 1 to producefrequencies within the voice or the wire carrier range. v

In the arrangement shown in Fig. 1, the tube 1, which forms part of theoscillator, has its output circuit connected with the antenna 2, but thesaid output circuit may be connected with a wire line as in Figs. 2 and3. Connected with and forming part of the output circuit is a resonantcircuit made up of the inductance 3 and the variable condenser 4. Thesaid inductance has its midpoint connected to ground through thecondenser 5. A balancing condenser 6 is effectively connected betweenthe grid and the plate of the oscillator tube 1. A resistance 7, whichmay be of the order of 5,000 ohms, is connected between the grid and thegrounded filament of the said tube. A source of plate potential 8 is connected with the midpoint of the inductance 3 to apply potential to theplate of the tube 1. The tube 9, which forms part of the amplifymgcircuit, has its plate connected with the grid of the oscillator 1, theconnection including a condenser 10 whose function is to keep the platepotential applied by the source 8 to the plate of the tube 9 fromafl'ectin the grid of the tube 1. The output circuit 0 the tube 9 isconnected across the grounded resistance 11 so that the potential toground across this. resistance is applied directly to the grid of theoscillator. The input circuit of the amplifier 9 is connected with aresonant control circuit comprising an inductance 12 and a variablecondenser 13. These elements are so put together as to constitute amechanically rigid tuned circuit of extremely low loss, which iselectrically shielded from outside disturbances by means of the shield14, and furthermore is rendered practically free from effects of changesintemperature. The inductance of this tuned circuit, which has its 10.

midpoint grounded, has two taps, on each side of the center of the coilabout a small fraction viz about one-tenth of the total number of turnsfrom the center. One of the ta s is connected with the grid of theamplifier; the other is connected through the balancin condenser withthe plate of the amp er, as shown. This prevents the resonant controlcircuit from oscillating of its own accord and it also raises the in utimpedance of the amplifier at the radio requencies involved. One side ofthe inductance 3 connected with the output side of the oscillator 1 andforming art of the resonant circuit connected thereis connected with thegrid of the amplifier 9, the connection includin a small variablecondenser 16. Ener is t us fed back from the free end of the inductancein the output circuit of the oscillator tube throu h the small variablecondenser 16 to the ri of the am lifier tube. This coupling con ensercontro s the magnitude of the oscillations fed back to the amplifier.

The method of operation of the system described above will probably bequite obvious from the foregoin description of the parts of the circuitan the functions that they perform, so that little further explanationshould be necessary. Oscillations produced by the tube 1 will cause theresonant circuit 34 to resonate at the frequency to which it isadjusted. A part of the energy at this freuency will be fed back throughthe conenser 16 to the conductor connecting the grid of the amplifier 9with one of the taps of the inductance 12 of the resonant circuit 12-13.This will cause the latter resonant circuit to resonate at the frequencyto which it is adjusted. This frequency will then be 0 impressed uponthe amplifier 9, which in turn will impress an amplified voltage of thisfrequency upon the input side of the oscillator 1 and thus the processwill be repeated. The major portion of the ener of the frequencygenerated will be transmitted throu h the condenser 17 to the antenna 2for ra iation therefrom. Only a small portion of the energy goesback'through the condenser 16 to the resonant circuit. It will thereforebe seen that the system described depends not on the stability of thevacuum tubes but upon the stability of the mechanically rigid resonantcircuit 1213. In other words, variations in the constants of the tubesas the operate will not produce any substantial e ect upon the tochanical viewpoint but is also shielded from extraneous sources ofinterference and is maintained at a substantially constant temrature. Itis also to be noted that the feedhiick condenser 16 which, for example,may

6; have a capacity of about 25 microfarads, may

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produce a slight effect upon the tuning of the resonant circuit 1213,but inasmuch as the ratios of the capacities 16 and 13 and the value ofthe coupling involved are low the effect of such coupling is negligible.nce the system is set 1n operation to produce a definite frequency itWlll continue to do so.

The arrangements shown in Figs. 2 and 3, in which the same referencenumerals have been used to indicate similar parts, are intended to showthe application of the invention to systems for the production ofoscillations within the range of frequencies applicable to wire carriersystems. Thus, in Fig. 2

the output circuit of the oscillator 1, which includes the inductance 3and the condenser 4, is coupled to an output circuit 20 by means of themutual inductance between the winding 3 and the winding 28 connectedwith the output circuit. The resonant control circuit 12-13, which isprotected by the shielding surface 14, is connected to ground throughthe midpoint of the inductance 12 and is also connected with the gridcircuit of the amplifier 9, the connection including a condenser 27. Asin Fig. 1, a feed-back connection is established from the inductance 3in the output circuit of the oscillator to the resonant circuit, theconnection including a condenser 16. This feed-back circuit is connectedto the input of the first am lifier at a point between the condenser 27an the grid of the amplifier 9. Another amplifier 21 is inserted betweenthe amplifier 9 and the oscillator 1 and the output of each amplifier isimpressed across a resistance such as 11 and 23, to which a source ofplate potential 24 is also connected. Condensers 10 and 29 are placed inthe grid circuits of the amplifier 21 and the oscillator 1 in order toprotect the rids of these tubes from the effect of the pfite batter 24.

the arrangement shown in Fig. 3, the resonant circuit in the output sideof the 0s cillator 1 is connected with the resonant control circuit 1213by means of a connecting circuit including the resistance 25 and thecondenser 26. As in Fig. 1, the feed-back coupling between the resonantcircuit in the output side of the oscillators of Figs. 2 and 3 is ofsuch magnitude as to reduce to a minimum the energy fed back to thecontrol circuit. In this manner the effect of external changes on thefrequency determined by the resonant control circuit 12-13 is greatlyreduced.

While this invention has been disclosed in a particular form it isapparent that it is caable of embodiment in other and different ormswithin the spirit and scope of the appended claims.

What is claimed is: 1. In a system for producing electrical oscillationsof constant frequency, the combination with a vacuum tube oscillatorhaving tween the output of the said oscillator and .nected with the in aresonant circuit connected with the output side thereof, of an amplifierconnected with the input side of said oscillator by a resistancecoupling, a sharply tuned resonant circuit shielded from externalinfluence conut side of the said amplifier, and means to eed back asmall amount of energy form the said first resonant circuit to the saidsecond resonant circuit.

2. In a system for producing electrical oscillations of constantfrequency, the combination with a vacuum tube oscillator having itsoutput circuit tuned to the desired frequency, of a control circuitsharply tuned to the desired frequency, shielding means to protect thecontrol circuit from external interference, a condenser couplingbetween-the saidfirst tuned circuit and the said control circuit to feedback a small amount of energy to energize the said control circuit, andmeans to connect the said control circuit 'to the input circuit of saidoscillator, the said connecting means including a unilateral devicepreventing undesired reaction between the said oscillator and the saidcontrol circuit, and a resistance coupling between the said oscillatorand the said unilateral device.

3. In a system for-producing electrical oscillations of constantfrequency, the combination with an oscillator having a tuned outputcircuit, of'an amplifier, means to connect said amplifier to the inputside of the said oscillator, the said connection including a resist-.

ance coupling, a second tuned circuit mechanically rigid and shieldedfromexternal interference connected with the input side of theamplifier, and a condenser coupling bethe second tuned circuit.

4. In asystem for cillations of constant requency, the combination withan oscillator having a tuned output circuit, of a frequency controlcircuit sharply tuned, mechanically rigid and shielded from externalinterference, means to couple the output circuit of the said oscillatorwith the said frequency control circuit, and means to connect the saidfrequency control circuitto the input of the said oscillator, the saidconnecting means comprising a unilateral device and a resistancecoupling between the said device and the said oscillator.

In testimony whereof, I have signed my name to this specification this21st day of September 1925. 1

RUSSELL S. OHL.

producing electrical os-

